The investigation of chemical structure of coal macerals via transmitted-light FT-IR microspectroscopy

The Late Permian coals from South China are characterized by high content of the unique maceral "barkinite" and elemental hydrogen, typically produce high yields of tar, and exhibit significant oil-producing potential. "Barkinite" has been identified as suberinite for a long time, but now many Chinese workers have concluded that "barkinite" is not suberinite, rather it is a distinct maceral. The term "barkinite" was formally certified and named by the State Bureau of Technical Supervision of PR China in 1991, 1995, and 1999, however, it has not been recognized as a scientific term by international coal geologists and the ICCP. Transmitted-light FT-IR microspectroscopy and curve-fitting analysis were used to investigate the chemical structure of "barkinite"; at the same time, parallel studies were also carried out on vitrinite, fusinite, and sporinite. In comparison with other maceral types, the micro-FT-IR spectra of "barkinite" are characterized by very strong intensities of aliphatic CH(x) stretching vibrations at 3000-2800 cm(-1) and deformations at 1460-1450 cm(-1), less intense bands from aromatic CC ring stretching at 1610-1600 cm(-1) and aromatic CH out-of-plane deformations at 900-700 cm(-1). The aliphatic side-chains in the molecular structure of "barkinite" are longer and less branched. In addition, there also appear intense aliphatic ether COC and alcohol CO stretching bands at 1100-1000 cm(-1), notable aromatic CH stretching vibrations at 3050-3030 cm(-1), weak OH stretching bands of water in the 3600-3200 cm(-1) range, and rare acid CO group absorption at 1740-1700 cm(-1). Collectively the IR spectral characteristics indicate that "barkinite" is composed of great numbers of long chain aliphatics, a fewer of aromatics and rare of oxygenate components. The chemical structure of "barkinite" show that the high tar yields and the relatively high oil-producing potential of the Late Permian coals from South China are attributable to the high "barkinite" content in the Leping coals and the great numbers of long chain aliphatic substances contained in this maceral.